DEAR DOCTOR K: I was recently diagnosed with prostate cancer. My doctor says my cancer is slow-growing and that we should just monitor it for now. Why not treat it right away?

DEAR READER: I know this will sound odd, but cancer is not always bad for your health. There are types of cancer that can cause no symptoms, that grow slowly (if at all) and that are unlikely to spread. There are types of cancer that you will never know you had. You will die with these cancers, but you won't die from them.

Prostate cancer is a good example. Many men die with, but not from, prostate cancer. The autopsy shows tiny cancers in their prostate. Sometimes, these cancers can't even be seen with the naked eye, but only under a microscope.

So while it's natural to want to rid your body of cancer as quickly as possible, that's not always the best option. If you get treatment for a cancer that never is going to harm you, then the treatment could be worse than the disease.

That's why when a person is diagnosed with prostate cancer, the doctor tries to determine how likely it is to harm you. The doctor studies the piece of the cancer that has been removed by the biopsy.

Traditionally, doctors have used something called the Gleason score to describe how aggressively your cancer is growing. The higher your Gleason score, the faster the cancer cells are multiplying. Doctors also are starting to study which genes are turned on in the cancer. This is another way of estimating how aggressive the cancer will be.

Your doctor also has tried to determine if the cancer has spread and, if so, how far. This is called "staging" the cancer. Your prognosis is based on the stage of your cancer at the time of diagnosis. Your doctor has recommended having no treatments now, and just watching the cancer closely with repeated tests. This means your tumor does not look aggressive, and there is no evidence that it has spread.

If you have a non-aggressive type of prostate cancer that has not spread, your age is a factor in deciding on treatment. The older you are, the more likely you are to die with but not from the cancer. Slow-growing prostate cancers can take 15 to 20 years or more to grow. You're more likely to die from another condition.

Even if your cancer is unlikely to harm you, why not just have surgery to take it out? Because surgery and some non-surgical treatments involve risks, such as erectile dysfunction and urinary incontinence.

Men like you often choose active surveillance (also called "watchful waiting"). This strategy involves monitoring the cancer closely and regularly. If the cancer advances or becomes more active, you can pursue more aggressive treatment.

On the other hand, if you tend to worry a great deal, you might prefer treatment to waiting and worrying -- even if your tumor is slow-growing. Just be aware of the risks of treatment.